A nightmare is a dream that can cause a strong negative emotional response from the sleeper, typically fear and/or horror. The dream may contain situations of danger, discomfort, psychological or physical terror. Sufferers usually awaken in a state of distress and may be unable to return to sleep for a prolonged period of time.

Nightmares can have physical causes such as sleeping in an uncomfortable or awkward position, having a fever, or psychological causes such as stress and anxiety. Eating before bed, which triggers an increase in the body's metabolism and brain activity, is a potential stimulus for nightmares.

Occasional nightmares are commonplace, but recurrent nightmares can interfere with sleeping patterns and cause insomnia that may require medical help. Recurring post-traumatic stress disorder nightmares in which real traumas are re-experienced respond best to a technique called imagery rehearsal. First described in the 1996 book Trauma and Dreams by Harvard psychologist Deirdre Barrett, imagery rehearsal therapy involves the dreamer coming up with an alternate, mastery outcome to the nightmare, mentally rehearsing that outcome awake, and then reminding themselves at bedtime that they wish this alternate outcome should the nightmare recur. Research has found that this technique not only reduces the occurrence of nightmares and insomnia, but also improves other daytime PTSD symptoms.